The present invention relates to an integratable power transistor with optimization of direct secondary breakdown phenomena. In particular, the present invention relates to a power transistor wherein the emitter area is divided into a plurality of regions surrounded by base and collector regions which extend mutually parallel so as to form a plurality of elementary transistors or "fingers".
As known, one of the main problems associated with both integrated and discrete power transistors is linked to the phenomenon of direct secondary breakdown.
A typical plot of the curve (which traces the maximum collector current as a function of the voltage between the collector and the emitter) within which the transistor cannot suffer damage is shown in FIG. 1 and is indicated by A. The area delimited by said curve, which is known as SOA (Safe Operating Area), is limited upward by the maximum current which can be delivered by the collector (line 1) and then by a line, termed "second breakdown" line (line 4), which is destructive for the transistor although the power dissipated thereby, along said line, decreases as V.sub.CE increases. For example, in the case of a transistor which can carry, at 20 V of V.sub.CE, a current of 4 A (and therefore dissipates a power equal to 80 W), the maximum collector current at 50 V can decrease to values around 200 mA, with a dissipated power of 10 W.
This phenomenon is due to the fact that the various regions of the power transistor are not fed uniformly: a variation in the base-emitter drop (V.sub.BE) along the power transistor can cause an increase in the current which flows in a certain region, which will therefore tend to heat more than the others. Consequently, since V.sub.BE decreases as the temperature rises (by approximately 2 mV/.degree.C.), the hottest region tends to conduct even more current, triggering a phenomenon which can be regenerative and leads to the breakdown of the power transistor in a narrow region thereof. In practice, the problem of second breakdown is due to the fact that in every finger there is a region (usually the central one) which is hotter, in which the above described phenomenon can occur and in which high temperatures (300.degree.-400.degree. ) can be reached. Consequently even a single finger can undergo a second breakdown.
In order to obviate this disadvantage, an attempt has been made to distribute the current in the transistor as uniformly as possible.
A known solution in this sense consists in providing so-called "ballast" emitter resistors. This solution is shown by way of example in FIG. 2, which illustrates the equivalent circuit diagram of a power transistor of the NPN type. As can be seen, the transistor is composed of a plurality of elementary transistors Q.sub.A, Q.sub.B, Q.sub.C etc. which are connected in parallel, and a resistor R.sub.E is inserted between the emitter of each elementary transistor and a common point (which defines the emitter of the power transistor).
This structure allows to obtain an improvement in the SOA see curve B of FIG. 1, which again comprises a constant-current line--line 1--, a constant-power line--line 2--along which the transistor can be damaged due to the increase in temperature caused by dissipation, and the second breakdown line--line 3--) and in particular it is useful when the currents which flow in the transistor are relatively high, but for high voltages between the emitter and the collector (V.sub.CE) the critical collector current (I.sub.C) can be low, so that the voltage drop on the ballast resistor R.sub.E is also low and therefore the effectiveness of this system tends to disappear.
Especially in integrated circuits, said known structure is furthermore executed as shown in FIGS. 3 and 4, wherein the ballast resistors are formed by the emitter itself. In particular, said figures illustrate a portion of an elementary transistor (or "finger") in which 5 indicates the collector area, 6 indicates the base area and 7 indicates the emitter area. The base region 6 is executed so as to be unbroken, surrounds the emitter area 7 and furthermore extends inside said emitter area (regions 6' which are not diffused during the execution of the emitter regions) so as to form narrow strips 8 which define the ballast resistors. FIG. 3 furthermore illustrates the collector metallic layer (or "metal") 9, the contours whereof are indicated in broken lines, the base metallic layer 10 (dot-and-dash lines) and the emitter metallic layer 11 (broken lines), as well as the collector contacts 12, the base contacts 13 and the emitter contacts 14. Consequently, a part of the emitter (indicated by 15 in FIG. 4) which is arranged directly below the contact and is therefore highly activated, is free from the ballast resistors, so that the actual equivalent circuit diagram becomes the one illustrated in FIG. 5, wherein each cell of the power transistor Q.sub.A, Q.sub.B etc. is replaced with a pair of transistors Q'.sub.A, Q".sub.A and respectively Q'.sub.B, Q".sub.B, etc. This fact further reduces the effectiveness of this solution.
Other structures for increasing second breakdown are furthermore also known; however, they are more complicated, since they require particular drivings of the power transistor and therefore additional components which increase the area occupation and which therefore should desirably be eliminated.